Both Washington and Seoul - quoting the Paris-based World Organisation for Animal Health, which classifies the US as a country of "controlled BSE risk" - insist that US beef is now safe.

But protesters disagree.

According to a deal agreed by Seoul and Washington in April to allow the beef trade to resume, Seoul agreed to abide by an American BSE inspection scheme. Under this scheme, 1% of US cattle are inspected each year - a proportion protesters claim is too low to offer adequate safety guarantees.

South Korea has also sent back several shipments of US beef because they contained bone fragments, which are at particular risk of carrying BSE.

Protesters accuse US beef producers, backed by some lawmakers, of pressing to remove Seoul's right to send back such beef.

But there are other areas of concern too. In a country which has hosted tens of thousands of US soldiers since the 1950-53 Korean war, underlying the issues about food sovereignty are long-standing worries about South Korea's political sovereignty.

Some see the beef import deal - agreed hastily before President Lee's first trip to Washington - as a humiliating concession to Washington, which compromised public health standards in order to win favour from Korea's superpower backer.

Does the anger extend to issues beyond the beef imports?

It depends on which protesters you speak to. These protests have been notable for the variety of people who have joined them - school students, office workers, parents, unionists, reservist soldiers in their uniforms and activist groups of many different political stripes.

The protests have mobilised a broad spectrum of participants

Some protesters say their sole concern is the safety of US beef. But others call the beef issue the final straw.

Since taking office, Mr Lee has floated controversial plans to build a large industrial canal running the length of the country. His personnel appointments have attracted criticisms of cronyism. His plans to privatise swathes of the public sector have angered many.

Some protesters have accused Mr Lee of arrogance - such as when the president referred to himself as the CEO of Korea. "What does that mean?" one protester asked. "Are we his employees?"

The biggest rally so far - for which police say 100,000 people attended, while organisers claim a turnout of up to one million - was on 10 June, the 21st anniversary of massive street protests in 1987, now seen as pivotal in the country's turn towards democracy.

The date was symbolic. Some protesters see the conservative President Lee as representing a return to South Korea's authoritarian past - and the beef protests as vital in resisting this.

Whatever is motivating protesters, the demand for President Lee to stand down - and after less than four months in office - is now a common slogan of the protests.

What have Seoul and Washington done to try to resolve the crisis?

Amid the political furore, South Korea delayed the date on which beef imports were supposed to resume.

Neither Seoul nor Washington said it was possible to renegotiate from scratch the agreement that re-opened South Korea's doors to US beef.

South Korea said such a move could jeopardise a recently agreed free trade agreement (FTA) between the two sides, as US deputies have tied ratification of the FTA to renewed beef exports.

Instead, South Korea pushed for another solution: a voluntary agreement by US beef producers not to export beef from cattle over 30 months old (thought to be particularly susceptible to BSE).

But it is unclear whether the adoption of this voluntary ban will placate the protesters, many of whom are calling for complete renegotiation of the beef agreement.

How serious is this crisis for President Lee?

This is a very serious crisis, according to the BBC's John Sudworth in Seoul - reflected in the offer by Mr Lee's whole cabinet, including the prime minister, to resign over the issue.

His administration has also faced paralysing strikes from long-haul lorry drivers, construction industry workers, and up to 600,000 members of the militant Korean Confederation of Trade Unions who oppose his plans for privatisation and pension reform.

Mr Lee could also face a backlash over worsening economic conditions in Korea.

The president vowed to bring economic growth of 6% to the country in 2008 - but instead the country is battling a global downturn and surging inflation.

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